A composite number is a number that can be divided by numbers other than 1 and itself.
Think of it like a building made out of Legos. If you have a building made of one Lego block, it's not a very big building, and it's only made up of one piece, so it's not a composite number. But if you have a building made of many Legos, then it's a composite number because it can be taken apart into many pieces, or divided.
The numbers that can only be divided by 1 and themselves are called prime numbers. So, if a number is not a prime number, it's a composite number. Examples of composite numbers are 4, 6, 8, 9, and 10.